1. Field of the Invention
The present invention relates to a simplified process for controlling the color-shifting properties of chiral nematic (also called cholesteric) liquid crystal polymers. In particular, the invention relates to controlling the color-shifting properties of a chiral liquid crystal polymer by incorporating therein one or more salts which are capable of changing the position of the selective reflection band (color) of the polymer. Correspondingly modified chiral liquid crystal polymers are useful, for example, for marking purposes.
2. Discussion of Background Information
Counterfeit is no longer a national or a regional problem but a worldwide problem which has an impact not only on manufacturers but also on the consumer. Counterfeiting is a significant problem with goods like clothes and watches but becomes even more serious when it affects medicines and drugs. Each year thousands of people around the world die because of counterfeit drugs. Counterfeiting has also an impact on government revenues in that it affects the collection of taxes for, e.g., cigarettes and alcohol because of the existence of a black market where it is impossible to track and trace counterfeit (smuggled, diverted, etc.) products with no valid tax stamps.
Many solutions have been proposed to make counterfeiting impossible or at least very difficult and/or costly, for example RFID solutions and the use of invisible inks.
More recently, a security feature has emerged and is used to authenticate a genuine product such as a drug and to avoid counterfeiting. This technology is based on optically variable inks. Its principle is based on the difference in observable color of a marking made with an optically variable ink when a packaging, security document, etc. carrying the marking is viewed from different angles (“viewing-angle dependent color”).
Optically variable inks provide first-line recognizability not only by a person, but also facilitate machine-readability. Many patents describe this security product, its composition and its application. One example of the many types of optically variable inks is the class of compounds called cholesteric liquid crystals. When illuminated with white light, the cholesteric liquid crystal structure reflects light of a certain color which depends on the material in question and generally varies with the viewing angle and the temperature. The cholesteric material itself is colorless and the observed color is the result of a physical reflection effect at the cholesteric helical structure that is adopted by the liquid crystal precursor composition at a given temperature. See, e.g., J. L. Fergason, Molecular Crystals, Vol. 1, pp. 293-307 (1966), the entire disclosure of which is incorporated by reference herein.
EP-A-1 381 520 and EP-A-1 681 586, the entire disclosures of which are incorporated by reference herein, disclose a birefringent marking and a method of applying the same in the form of a liquid crystal layer having a non-uniform pattern of regions of different thickness. The applied liquid crystal coating or layer may provide for a hidden image on a reflected substrate, which image is invisible when viewed under non-polarized light but is rendered visible under polarized light or with the help of a polarization filter.
U.S. Pat. No. 5,678,863, the entire disclosure of which is incorporated by reference herein, discloses means for the identification of documents of value which include a paper or polymer region, said region having a transparent and translucent characteristic. A liquid crystal material is applied to the region to produce an optical effect which differs when viewed in transmitted and reflected light. The liquid crystal material is in liquid form at room temperature and must be enclosed in a containing means such as microcapsules in order to be suitable for use in a printing process such as gravure, roller, spray or ink jet printing.
The ordered liquid crystalline state depends upon the presence of a chiral dopant. Nematic liquid crystals without chiral dopant show a molecular arrangement that is characterized by its birefringence. Nematic polymers are known from, e.g., EP-A-0 216 712, EP-A-0 847 432, and U.S. Pat. No. 6,589,445, the entire disclosures of which are incorporated by reference herein.
As mentioned above, the liquid crystal based security feature provides first-line recognizability by the consumer and also by retailers and producers of goods and articles. Like for many other security features which are used in the market, there is always the temptation for counterfeiters to reproduce these security features and therefore misleads consumers and retailers. In view of the foregoing facts, there continues to be a need to improve the security of liquid crystal polymer materials based on liquid crystal precursors. The ability of manufacturing security features with tight quality standards is an important deterrent against counterfeiting, as it demands a significant effort on the part of the counterfeiter to produce a fake of comparable quality. In the case of chiral liquid crystal polymers it is then highly desirable to obtain as exactly as possible the specified color-shifting properties, routinely and reliably.
The color-shifting properties of a chiral liquid crystal polymer are closely connected to its composition and in particular, to the ratio of nematic and chiral dopant molecules present in the polymer. It is difficult to obtain a specific and controlled color shift to a predetermined wavelength, as a number of parameters (temperature, nature of substrate, etc.) affect the color shift obtained upon applying a chiral liquid crystal precursor composition onto a substrate. An existing system sold under the name Paliocolor© by BASF uses the concentration of the chiral dopant in the mixture of nematic compound and chiral dopant to obtain the desired reflection wavelength. In the field of security marking and/or coding, the marking or coding is often part of the production line process, for example, to protect packaging, articles and goods, banknotes, security documents, etc. Very often, during the production line process, it is necessary to modify the initial composition, in order to control the properties of the resulting marking and/or coding (in particular, the position of the selective reflection band). Once the mixture is prepared, one can make a correction to shorter wavelengths, by adding the appropriate quantity of chiral dopant. On the contrary, making a correction to longer wavelengths is impractical, as it requires adding a substantial amount of nematic compound. Thus, the fine-tuning of the composition leading to a specific and desired reflection wavelength is a delicate step, often resulting in an important waste of time and/or material.
In view of the forgoing facts, using a chiral liquid crystal precursor composition in a production line (processes for making items such as passports, packaging, etc.) in order to mark packaging with, e.g., a data matrix and/or generating codes within a short period of time often creates several problems. Typical problem that may be encountered is for example, obtaining in a reproducible manner the desired optical properties of the chiral liquid crystal polymer, and therefore it is desirable to have available means to adjust these optical properties. It is also problematic during the production line processes to apply with a printing station several different colors on packaging by adding the chiral dopant to an existing composition in a controlled manner. Therefore there is a need to find alternative and efficient solutions in a production line (processes for making items such as passports, packaging, etc.) for quickly obtaining a modification (or a single stable color) of color-shifting properties of a chiral liquid crystal precursor composition without encountering the problems set forth above.